Reference oscillator with variable duty cycle, frequency synthesizer and signal receiver with reference oscillator

Abstract

A reference oscillator for a transmitter and/or a receiver of electromagnetic signals. The reference oscillator is suitable for generating a modified reference signal alternating ON times and OFF times with a predefined duty cycle from a signal supplied by a reference resonator. The reference oscillator also includes an adjustment circuit suitable for adjusting the duty cycle of the modified reference signal according to at least one adjustment parameter dependent on a rank of at least one harmonic component of the modified reference signal so as to minimize at least one harmonic component of the modified reference signal. A frequency synthesizer and a radio frequency signal receiver can include such a reference oscillator.

The invention claimed is: 
     
       1. A reference oscillator for a transmitter and/or a receiver of electromagnetic signals, the reference oscillator being suitable for generating a modified reference signal alternating ON times and OFF times with a predefined duty cycle from a signal supplied by a reference resonator, wherein the reference oscillator comprising:
 an adjustment circuit suitable for adjusting the duty cycle of the modified reference signal according to at least one adjustment parameter so as to minimize at least one higher harmonic component of the modified reference signal, injected parasitically into a spectrum of electromagnetic signals, and 
 a reference generator coupled to the resonator for producing an initial reference signal, the adjustment circuit being coupled to the reference generator for generating the modified reference signal with a predefined duty cycle from the initial reference signal, 
 wherein the adjustment circuit generating the modified reference signal comprises:
 a phase-locked loop or PLL, connected to the reference generator and suitable for producing a signal of frequency n times greater than the frequency f ref  of the initial reference signal, 
 a counter connected to the PLL loop for counting the pulses of the signal produced by the PLL loop, 
 a comparator for comparing a number of pulses counted to a predefined value m, and producing an ON signal if the number of pulses counted is less than the predefined value m or an OFF signal otherwise, and 
 a pulse generator connected to the reference generator and arranged for producing an initializing signal of the same frequency as the initial reference signal, said initializing signal being applied to an initializing input of the counter. 
 
 
     
     
       2. The reference oscillator according to  claim 1 , wherein the adjustment circuit also comprises a circuit for synchronizing the modified reference signal on one flank of the initial reference signal. 
     
     
       3. The reference oscillator according to  claim 2 , wherein the synchronization circuit comprises an RS flip-flop, a SET input of which is connected to the pulse generator and a RESET input of which is connected to the output of the comparator, the synchronized oscillating signal being available at an output of the flip-flop. 
     
     
       4. The reference oscillator according to  claim 1 , also comprising a Man/Machine interface for enabling a user to supply the adjustment circuit with at least one adjustment parameter of the duty cycle. 
     
     
       5. A frequency synthesizer comprising a reference oscillator according to  claim 1  and a frequency matching circuit for producing a high frequency oscillating signal. 
     
     
       6. The frequency synthesizer according to  claim 5 , wherein the frequency matching circuit comprises a PLL loop arranged for receiving the modified reference signal of frequency f ref  and producing an oscillating signal of higher frequency x·f ref . 
     
     
       7. The frequency synthesizer according to  claim 6 , wherein the frequency matching circuit comprises:
 a voltage controlled oscillator VCO for supplying the high frequency oscillating signal OUT, 
 a divider circuit for dividing the frequency of the signal OUT by a factor N in order to supply a divided frequency signal f div , 
 a phase detector for comparing the modified reference signal at a reference frequency f ref  with the divided frequency signal f div  supplied by the divider circuit and supplying two control signals UP, DOWN according to the result of the comparison, 
 a charge pump for receiving the two control signals UP, DOWN of the phase detector, and 
 a low-pass filter for filtering an output signal of the charge pump for supplying a filtered control signal to the voltage-controlled oscillator. 
 
     
     
       8. A signal receiver, comprising:
 an antenna for receiving electromagnetic signals, 
 at least one low noise amplifier for amplifying the signals received by the antenna, 
 at least one reference oscillator, according to  claim 1 , for producing a modified reference signal or a high frequency oscillating signal with a predefined duty cycle, 
 a mixer for mixing the amplified and received signals with the modified reference signal or with the high frequency oscillating signal for generating intermediate signals, and 
 a filter for filtering the intermediate signals, 
 an adjustment circuit of the reference oscillator being suitable for adjusting the duty cycle according to a frequency of the electromagnetic signals received for minimizing the amplitude of at least one harmonic component of the modified reference signal and present in the filtered intermediate signals.